A jar contains 5 blue marbles, 8 red marbles, 4 white marbles, and 3 purple marbles.
Number of blue marbles = 5
Number of red marbles = 8
Number of white marbles = 4
Number of purple marbles = 3
Total number of marbles = 5 + 8 + 4 + 3
Total number of marbles =20
The probability formula is defined as the possibility of an event to happen is equal to the ratio of the number of favourable outcomes and the total number of outcomes

Probability of picking white marbles + Probability of not picking marbles = 1
Probability of not picking white marbles = 1 - Probability of picking marbles

As :
Probability of not picking white marbles = 1 - Probability of picking marbles
Probability of not picking white marbles = 1 - 0.2
Probability of not picking white marbles = 0.8
Answer : Probability of not picking white marbles = 0.8